1. Field of the Invention
The present invention relates generally to an opposed type optical scanning device capable of simultaneously scanning surfaces of plural photoconductors in a multi-color image forming apparatus such as a color laser printer, a digital color copier, etc., and more particularly relates to an arrangement of optical elements in the optical scanning device.
2. Discussion of the Background
An image forming apparatus includes an optical scanning device to optically scan a surface of a photoconductor to write an electrostatic latent image on the photoconductor. The optical scanning device is generally configured such that an optical beam from a light source is deflected by a rotating deflector, the deflected beam is formed into a beam spot on a surface of the photoconductor by a scanning optical system, and the surface of the photoconductor is scanned with the beam spot. Recently, in a multi-color image forming apparatus such as a color laser printer, a digital color copier, etc., a method of optically scanning surfaces of plural photoconductors simultaneously has been adopted to improve productivity of the apparatus. An optical scanning device adopting such a method of simultaneously scanning surfaces of plural photoconductors includes a scanning optical system for each of the plural photoconductors. Accordingly, the number of optical elements in the optical scanning device is proportional to the number of the photoconductors, so that the number of parts in the optical scanning device inevitably increases.
As an optical scanning device employing such a method of simultaneously scanning surfaces of plural photoconductors, a so-called opposed type optical scanning device performing optical scanning at both sides of a single rotating deflecting device is described in Japanese Patent Laid-open publication No. 2002-196271 and Japanese Patent publication No. 3124741.
In an opposed type optical scanning device configured to simultaneously scan surfaces of plural photoconductors, generally, as many scanning optical systems as the plural photoconductors are arranged in a single optical housing to perform optical scanning relative to surfaces of the plural photoconductors, respectively. The single rotating deflecting device is provided with dual reflecting surfaces (upper and lower reflecting surfaces) to decrease space, and scanning optical systems, which are independent from each other, are arranged relative to each of the upper and lower deflecting surfaces of the single deflecting device. Further, because the scanning optical systems are arranged independent from each other, in each of the scanning optical systems, one or more folding-back mirrors are arranged upstream of a scanning lens in the direction in which an optical beam travels. Such an opposed type optical scanning device using a single deflecting device is advantageous in cost when compared to an optical scanning device using a plurality of deflecting devices. However, the opposed type optical scanning device is still relatively expensive because as many scanning optical systems as plural photoconductors are necessary, and thereby as many optical elements as the scanning optical systems are necessary.
A scanning optical system of an optical scanning device includes optical elements such as an fθ lens as a scanning lens, plural folding-back mirrors, and a long lens (toroidal lens) as a scanning lens having power in a sub-scanning direction. The arrangement of these optical elements in the optical scanning device and the performance of the optical elements greatly influence the quality of an image formed by an image forming apparatus using the optical scanning device.
For example, in an optical scanning device simultaneously scanning surfaces of plural photoconductors, it is important to always keep uniform curvatures in scanning lines formed on the plural photoconductors. If geometrical characteristics of scanning lines formed on the plural photoconductors are not uniform among the plural photoconductors, when images formed on the plural photoconductors are sequentially transferred, for example, onto a recording sheet while being superimposed on top of another on the recording sheet, the images are not superimposed with each other correctly, so that the quality of a resulting image formed on the recording sheet deteriorates. In particular, in a color image forming apparatus in which latent images on plural photoconductors are developed with toner of different color from each other, due to color deviation caused by such deviations in scanning lines on the plural photoconductors, color reproducibility greatly deteriorates.
Generally, in a scanning optical system of an optical scanning device, a scanning lens having power in the sub-scanning direction, which is usually a long lens, has dominant influence on such scanning line curvature on a photoconductor. That is, in a scanning lens, when focus lines forming an optical axis center of the scanning lens are not parallel to an attachment side of the scanning lens, scanning line curvature is caused on a photoconductor. Focus line curvature in a lens is inevitably caused by a limit in processing the lens with molding, and if the focus line curvature in the lens can be decreased at all, it is likely that the cost of processing the lens will increase.
In particular, recently, resin lenses have been widely used because of such merits as low cost and that a freely curved surface can be formed. In a resin lens, however, due to internal distortion of the lens when molding the lens with a mold and unevenness in temperature of the mold, the above-described focus line curvature is more remarkably caused than in a glass lens.
In an opposed type optical scanning device, it is also important to make uniform characteristics of beam spots on plural photoconductors. Even slight differences in the beam spot characteristics among the plural photoconductors can cause deterioration in the quality of a resultant image, for example deterioration in the color reproducibility and the color evenness. Here, the beam spot characteristics on a photoconductor include not only the beam spot diameter but also the beam strength (light quantity) and the beam spot position (imaging position) on the photoconductor, so that to make uniform the beam spot characteristics on plural photoconductors, it is important to perform uniform optical scanning relative to the plural photoconductors. Deterioration in the beam spot characteristics on a photoconductor may be caused, for example, by deformation at parts of an optical housing where optical elements are mounted, which may be caused by thermal expansion arising from a rise in temperature in the optical housing, and by deviations in beam incident positions relative to the optical elements.
It is also well known that in an optical scanning device, the beam spot diameter and beam spot position on a photoconductor are greatly influenced by a beam incident position relative to a scanning lens. Therefore, when a folding-back mirror is arranged upstream of the scanning lens as in the above-described opposed type optical scanning device, very high dimensional accuracy is required in the mounting surface of an optical housing where the folding-back mirror is mounted. This leads to increasing parts cost of the optical scanning device.